Capillary valves have turned out to be useful for controlling liquid transport in centrifugal-based microfluidic devices. One of the main advantages has been that neither this kind of valves nor centrifugal force requires mechanical means on-board a microfluidic device. Capillary valves are stops for a liquid flow/transport and are not to be confused with flow restrictions that permit flow but reduce the flow rates (impede flow). Once a liquid front is breaking through a capillary valve there is in principle no hinder for the ongoing flow or for restarting after a stop (as long as contact with liquid is maintained).
The use of capillary or surface tension stop functions in the form of valves, vents, anti-wicking means etc in centrifugal-based microfluidic systems has been described by Gamera Biosciences and Gyros AB, among others: WO 9853311, WO 0078455, WO 0187486, WO 0079285; WO 0187487; WO 2004058406; WO 9807019 (all of Tecan Trading/Gamera Biosciences) and WO 9958245; WO 0040750; WO 0147638; WO 0185602; WO 0274438; WO 0275312; WO 03018198; WO 03024598; WO 04103890; WO 04103891; etc (all of Gyros AB).
According to the Gamera/Tecan publications an increase in cross-sectional dimension to obtain a capillary valve in a hydrophilic microchannel could be anything from continuous to abrupt. Our work with microfluidic devices has primarily dealt with devices replicated in plastic material. It is our experience that capillary valving based on increases in cross-sectional dimensions requires extremely sharp and distinct changes of a kind not recognized in WO 9807019. Conventional embossing, injection moulding etc and other replication techniques thus seem insufficient for the manufacture of microfluidic valves based on a change in cross-sectional dimension. In order to distinguish changes that create a valving effect from other changes the former will be called “sharp” increases/changes compared to other changes that have no or only an insignificant valving effect.
Another centrifugal based approach is given by the company Abaxis. See for instance U.S. Pat. No. 5,186,844, U.S. Pat. No. 5,242,606, U.S. Pat. Nos. 5,693,233, 5,160,702 etc and J. Autom. Chem. 17(3) (1995) 99-104 (Schembri et al). In Abaxis' system the flow resistance in a channel going between the reservoirs controls the flow between an inner reservoir and an outer reservoir. Compare also U.S. Pat. No. 6,632,656 (Gyros AB). In some instances the flow between an inner and an outer reservoir is controlled by so called siphons, i.e. the channel concerned is of capillary dimensions starts from the inner reservoir by making an inward turn (elbow) before ending in the outer reservoir. At a sufficiently high spin speed centrifugal force will prevent liquid from passing over the extreme of the elbow. When the spin speed is slowed down and/or stopped wicking starts transporting liquid over the extreme. Resumed spinning further supports this liquid transport. An inner reservoir may be designed as a separation unit for separating off suspended particulate material, such as cells, from a liquid, such as blood. In order to safely retain the particulate material in the reservoir the bottom of the of the reservoir has weirs delineating the outer part from the inner part of the reservoir such that the particulate material will be maintained in the outer part even when spinning is decreased or stopped. Mixing can be accomplished in mixing chambers containing two different aliquots by cycles of forward and reversed spinning or of accelerated and decelerated spinning.
In conclusion:                Gyros' system primarily contemplates processing nl-aliquots of liquid containing reagents by the use of centrifugal force and/or capillary force in hydrophilic microconduits. Starting aliquots containing an uncharacterized entity, e.g. an analyte, may be in the μl-range, such as ≦30 μl or ≦20 μl. Earth's gravity is as a rule not of interest.        Abaxis' system utilizes considerably larger volumes and dimensions, together with centrifugal force and gravity. The wettability of the channels and capillary force is of minor interest (except for the siphons discussed above). Starting aliquots containing an uncharacterized entity and reagent aliquots are typically well above the nl-range, such as ≧10 μl or ≧30 μl or ≧30 μl. In many cases the channels are large enough to permit entrance of liquid into non-vented reservoirs without risk for inclusion of air bubbles (filling and venting in parallel through the same channel).        Tecan's system is intermediary to Gyros' and Abaxis' systems.        